Titanium alloy products and methods for their production

ABSTRACT

A titanium alloy product having good tribological properties without the need to introduce an alloying element into the surface is produced by casting or casting and forging a titanium alloy consisting of 2 to 15% by weight silicon or 5 to 15% by weight nickel, 0 to 7% by weight of at least one strengthening element selected from aluminum, tin, zirconium, chromium, manganese, iron, molybdenum and niobium, and 0 to 2% by weight of a surface improving alloying element selected from boron, carbon, nitrogen, oxygen, and zirconium, the balance apart from impurities and incidental ingredients being titanium. Such alloy is then surface treated by surface melting and rapid solidification so as to produce a hard, wear-resistant surface layer without substantially affecting the bulk properties of the alloy. In another aspect, titanium alloy product which is resistant to both to rolling contact fatigue and to scuffing comprises casting or casting and forging a titanium alloy which is preferably of the above type, to the required product shape, deep surface hardening the resultant shaped product to a depth greater than 100 μm by localized re-melting without further alloying, optionally surface finishing (e.g., by machining, grinding, heat-treating or shot peening to the required final shape and/or surface finish, and forming on the intermediate surface a nitride or oxide or other surface film having a thickness which is not greater than 100 μm and which is resistant to scuffing.

This invention relates to titanium alloy products and methods for theirproduction, and in particular relates to such products which arerequired to have good tribological properties.

Although titanium is strong and light, applications of titanium ingeneral engineering are limited by its poor tribological properties. Ithas been proposed in, for example, WO 91/05072, EP-A-0246828, WO86/02868 and Metal Science and Heat Treatment, vol 26, no. 5/6, May-June1984, pages 335 and 336, to improve the tribological properties oftitanium and titanium alloys by melting suitable alloying ingredientssuch as boron, carbon, nitrogen, oxygen, silicon, chromium, manganese,iron, cobalt, nickel, copper into a surface layer using localised highenergy surface melting techniques such as laser beam melting or electronbeam melting. However, it is difficult to ensure that the requiredalloying ingredients are introduced evenly and in the correct quantityinto the melted surface layer. Additionally, it is difficult in anon-destructive test to check that the surface layer in the finalproduct has the correct distribution and composition.

It is an object of a first aspect of the present invention to obviate ormitigate the above disadvantage.

According to said one aspect of the present invention, there is provideda method of forming an titanium alloy product having a hardened layerthereon, comprising the steps of:

(1) forming the product (preferably by a casting operation and morepreferably by a casting and forging operation) from a titanium alloyconsisting of (a) 2 to 15% (preferably 5 to 9%) by weight silicon or 5to 15% (preferably 8 to 11%) by weight nickel, (b) 0 to 7% by weight ofat least one of the alloying elements conventionally used to strengthenwrought titanium alloys (aluminium, tin, zirconium, vanadium, chromium,manganese, iron, molybdenum and niobium) and (c) 0 to 2% by weight of atleast one alloying element added specifically for the purpose ofimproving the surface properties and selected from boron, carbon,nitrogen, oxygen and zirconium, the balance apart from impurities andincidental ingredients being titanium, and

(2) surface treating the product by a surface melting and rapidsolidification operation so as to produce a hard wear-resistant surfacelayer without substantially affecting the bulk properties of the alloy.

It has now been found that contrary to previous expectation, thetitanium-silicon alloy is quite easily forged at 1000° C. and so can bemade by casting and forging route, rather than having to cast it toshape. The use of a forging operation enables the structure of the alloyto be refined to permit an improvement in ductility of the bulk material(i.e., the core or substrate of the product as opposed to the surfacecase) by a sequence of working and heat treatment operations to producea wrought product. A typical sequence of such operations for an alloycontaining 8.5 wt % silicon would comprise casting an ingot, forging itat 1000° C. so as to produce an appropriately shaped billet or preform,annealing it at 550 to 750° C., precision die forging it at 1000° C. tothe required shaped component and machining it to approximate finaldimensions.

The surface treatment step (2) gives rise to a microstructural changeduring rapid cooling which results in a fine-grained surface layerconsisting predominantly of Ti-Si or Ti-Ni eutectic which issubstantially harder than the substrate.

It will thus be appreciated that there is no need to make specificadditions to the surface and that surface hardening takes placeautomatically upon surface melting as a direct result of the alloymaterial chosen.

With regard to the optional strengthening alloying elements and optionalsurface-improving elements, it will be noted that zirconium can be usedboth for strengthening and for surface-improving. In the case where itis included for both purposes, it will normally be present in an amountof up to 7% by weight.

Also according to said first aspect of the present invention, there isprovided a titanium alloy product, (preferably a cast or wroughttitanium alloy product), formed of a titanium alloy consisting of (a) 2to 15% (preferably 5 to 9%) by weight silicon or 5 to 15% (preferably 8to 11%) by weight nickel, (b) 0 to 7% by weight of at least one alloyingelement selected from aluminium, tin, zirconium, vanadium, chromium,manganese, iron, molybdenum and niobium, and (c) 0 to 2% by weight of atleast one further element selected from boron, carbon, nitrogen, oxygenand zirconium, the balance apart from impurities and incidentalingredients being titanium, the titanium in the bulk of the productbeing present predominantly in the α phase, and said product having alayer thereon containing fine grained Ti-Si or Ti-Ni eutectic.

In the case of Ti-Si, the eutectic is a Ti/Ti₅ Si₃ eutectic. In the caseof Ti-Ni, the eutectic is a Ti/Ti₂ Ni eutectic.

It has been proposed by Mazur, V. I. et al, "Cast and "Sintered Ti-SiAlloys", and by Bankovsky O. I. et al "Mechanical Properties of Ti-SiCermets", pages 141-146 and 435-440 of Proceedings of InternationalConference on "Processing and Properties of Materials", Birmingham, UK,September 1992 (Ed. M H Loretto), provide titanium alloys havingimproved mechanical properties such as high-temperature-strength andheat-resistance using powder metallurgy techniques where droplets oftitanium-silicon alloy are formed and rapidly cooled to form granules orgrains which are then hot isostatically pressed to form high strengthmaterials. However, such forming techniques are relatively complicatedand expensive and do not involve localised surface re-melting as in thepresent invention to develop a hardened layer whilst retaining arelatively tough core or substrate.

Where silicon is used in the alloy, the silicon content of the alloy ispreferably 7.5 to 8.5%, and most preferably is 8.5% by weight.

In a second aspect of the present invention, it is an object to improvethe tribological properties of titanium alloy in terms of both rollingcontact fatigue resistance and resistance to scuffing. This isparticularly important for products such as gears or bearings where thesurface is subjected to high contact loads and Hertzian stresses aregenerated below the surface which reach a maximum distance below thesurface. To withstand these stresses, it is generally accepted that ametallic material needs to be case hardened to a depth of about twicethe depth of maximum shear stress. In practice, this means case depthsof 200 to 1000 μm. It is generally accepted that such depth of hardeningcannot be achieved in titanium alloys except by molten phase surfacealloying. One proposed way of effecting this is by so-called "laser gasnitriding" which is a surface alloying process in which nitrogen isadded to the molten pool during laser beam melting of the surface. It isalso known from EP-A-0246828 to melt-harden the surface of a titaniumalloy by spraying the surface with a plasma jet containing, as a workinggas, a mixture of an inert gas and a hardening gas formed of one or moregases selected from nitrogen, carbon dioxide, carbon monoxide, oxygen,methane and ammonia, thereby melting the surface and alloying it withnitrogen, carbon, oxygen or hydrogen. In both of these methods, analloying addition is made to the surface material in order to harden it.

In accordance with said second aspect of the present invention, there isprovided a method of forming a titanium alloy product which is resistantboth to rolling contact fatigue and to scuffing, comprising the stepsof:

(a) forming a titanium alloy to the required product shape (preferablyby a casting or a casting and forging operation),

(b) deep surface hardening the resultant shaped product to a depthgreater than 100 μm by a technique involving localised surfacere-melting without further alloying,

(c) optionally surface finishing (eg by machining or grinding) to therequired final shape and surface finish,

(d) forming on the immediate surface a nitride or oxide or other surfacefilm having a thickness which is not greater than 100 μm (and usuallynot greater than 50 μm) and which is resistant to scuffing, and

(e) optionally performing a procedure such as shot peening or heattreatment after any of steps (b), (c) and (d) in order to modify theresidual stresses in the material and/or its other mechanicalproperties.

The deep surface hardening step (b) may be conducted simply by localisedsurface re-melting, e.g., by laser beam or electron beam, if thetitanium alloy used is a titanium-silicon or titanium-nickel alloy ofthe type used in the first aspect of the present invention. Thisprovides a surface resistant to deformation under high contact stresses.The titanium nitride or other surface film applied in step (d) providesa lower friction surface which is resistant to sliding wear andscuffing. The combination of steps (b) and (d) provides an ideal surfaceto resist the effect of combined rolling and sliding such as istypically encountered in gears and bearings.

EP-A-0246828 referred to above also discloses a process where a titaniumalloy is subjected to molten phase surface alloying by use of one ormore hardening alloy elements selected from aluminium, tin, boron, iron,chromium, nickel, manganese, copper, silicon, silver, tungsten,molybdenum, vanadium, niobium, columbium, tantalum and zirconium whichare included in the molten surface pool, whilst at the same timespraying the surface pool with a hardening gas such as nitrogen with thespecific objective of obtaining deep penetration of such hardening gasinto the molten surface layer with the intention that the final surfacelayer contains the hardening alloy element or elements and the hardeninggas or gases. The resultant final surface layer consists of a mixture ofmetallic phases (α and β titanium solid solutions) and intermetallic orcompound phases (such as Ti₂ Ni, TiN etc). Whilst EP-A-0246828 does notspecifically describe any machining or grinding subsequent to melthardening, it may be inferred from the reference therein to thepreparation of wear-resistant components such as poppet valves that somefinishing operation is needed in order to obtain the dimensionalaccuracy necessary for such components, for example on the seating faceof a valve. EP-A-0246828 does not however disclose any further surfacetreatment after final machining or grinding. By contrast, in the secondaspect of the present invention, step (d) is performed after any finalmachining or grinding (step (c)), in order to provide resistance toscuffing.

The thickness of the intermediate deep-hardened layer is preferably 200to 1000 μm, whilst the thickness of the nitride, oxide or other surfacefilm is preferably no more than 100 μm, more preferably no more than 50μm, and most preferably 1 to 20 μm.

Formation of the nitride or oxide or other surface film in step d) ofthe process may be effected by a variety of means. One preferred methodis the plasma thermochemical reaction process known as plasma nitridingin which the component is reacted with nitrogen in a low dischargeplasma in order to form layers of nitride and nitrogen-rich titanium onthe surface. Another preferred process is thermal oxidation in which thecomponent is heated in air at 600° to 850° C. to produce layers of oxideand oxygen-rich titanium on the surface. However it is also within thescope of the present invention to deposit a discrete compound layer onthe surface, for example by Physical Vapour Deposition. Such a compoundlayer may be titanium nitride or it may be aluminium nitride ortitanium-aluminium nitride or chromium nitride or alternatively a filmof oxide, carbide or boride.

The surface finish resulting from the surface re-melting operation isgenerally inadequate for use in a wear-resistant application and acomponent will normally be given a surface finishing treatment such asmachining or grinding to produce a smooth surface. In the second aspectof the present invention, this surface finishing may be carried outbetween steps (b) and (d) thereby retaining the scuff resistant lowfriction film produced by step (d) on the final surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph indicating the surface hardness Hv₀.1 for fourtitanium-silicon alloy samples which have been cast and subsequentlyelectron beam surface melted,

FIG. 2 is a graph plotting microhardness, Hv₀.1, against distance fromthe surface in respect of the four samples indicated in FIG. 1,

FIG. 3 is a graph similar to FIG. 2 for a Ti-8.5%Si alloy subjected toelectron beam surface melting at three traverse rates,

FIG. 4 is a graph similar to FIG. 2 for three titanium-nickel alloysamples, and

FIG. 5 is a block diagram showing the wear rate (mg/m) for varioussamples.

In one series of tests, small ingots or "buttons" were produced bymelting samples of titanium-silicon alloy as set out in Table 1 below ina water-cooled copper hearth and allowing them to cool on the hearth.

                  TABLE 1                                                         ______________________________________                                        Sample No.       Composition (% by wt)                                        ______________________________________                                        1                93% Ti, 7% Si                                                2                91.5% Ti, 8.5% Si                                            3                88% Ti, 12% Si                                               4                85% Ti, 15% Si                                               ______________________________________                                    

The as-cast Ti-Si buttons had a surface hardness of about 350 Hv₀.1 ascompared with a surface hardness of about 220 Hv₀.1 for an as-cast Tibutton containing no silicon. The buttons were then subjected toelectron beam surface re-melting using a Zeiss electron beam welderoperated at 100 kV with a current of 3 mA and a traverse rate of 16.4mm/s. The surface hardness and the microhardness profiles of the SampleNos. 1 to 4 are shown in FIGS. 1 and 2, respectively. It will be seenthat all samples produced a useful hardness increase as compared withthe as-cast buttons down to a depth of at least 500 μm, therebyeffecting case hardening down to a useful depth for articles to besubjected to high contact loads.

Sample 2 produced a better hardness result than Sample 1 and itsstructure was a finely divided eutectic mixture of alpha plus Ti₅ Si₃.Whilst Samples 3 and 4 had similar hardness, their structure consistedof relatively coarse dendrites of Ti₅ Si₃ in a matrix of eutectic. Thepresence of brittle dendrites would be likely to lead to poorermechanical properties, particularly fatigue properties and hence thecomposition of Sample 2 is preferred to that of Sample 3 or Sample 4.

From experimental work undertaken to date, the indications are thatuseful increases in hardness can be achieved by use of titanium alloyswherein the silicon content is greater than 5 wt % but not exceeding 9wt %. The ideal is an alloy having a silicon content of 8.5 wt % sincethis represents the eutectic composition. However, compositions up to 9wt % of silicon, i.e. slightly hypereutectic, are considered to beuseful in that the production of relatively coarse hard dendrites ofprimary Ti₅ Si₃ can be kept to within manageable proportions as far asthe fracture behaviour of the final product is concerned.

In another series of tests to demonstrate the present invention, SampleNos. 5 to 7 were prepared and subjected to microhardness profiletesting. The results are illustrated in accompanying FIG. 3. Sample No.5 corresponds to a Ti-8.5%Si alloy which has been subjected to electronbeam surface hardening without any alloy additions using a traverse rateof 16.4 mm/s. Sample Nos. 6 and 7 correspond to samples of the samealloy as used in Sample No. 5, but where the electron beam has beentraversed at a rate of 13.1 mm/s and 7.14 mm/s, respectively. The depthof molten pool is similar in the three cases, but the extent ofhardening can be varied by altering the traverse rate. The greatesthardening was achieved with the highest rate of traverse because it isbelieved) of the consequent more rapid quenching of the molten metal.

As a further example of the first aspect of the present invention,samples of Ti-Ni alloy buttons were prepared having the followingcompositions (by weight):

    ______________________________________                                        Sample No.       Composition (% by weight)                                    ______________________________________                                        8                Ti-7% Ni                                                     9                Ti-10% Ni                                                    10               Ti-28.5% Ni                                                  ______________________________________                                    

The surface of each button was ground flat and then surface re-melted byelectron beam using the same conditions as for Samples 1 to 4. Thehardness profiles through the re-melted surface of these samples areshown in FIG. 4.

Sample No. 9 is the known hypoeutectic composition and the re-meltedsurface metal had a fine α structure and a hardness in excess of 650 Hv.Beneath the remelted layer, the substrate structure was much coarserbecause of its lower rate of cooling and had a hardness of only about240 Hv. Sample No. 8 had a lower nickel content and the smaller volumefraction of the Ti+Ti₂ Ni eutectic microstructure gave rise to a lowerhardness. Sample No. 10 was a eutectic alloy having a wholly eutecticstructure of intermetallic compound Ti₂ Ni and α-titanium. The presenceof this amount of compound can be expected to result in poorermechanical properties, particularly fatigue properties, in the same wayas in the hypereutectic Ti-Si alloys. Furthermore, the high nickelcontent resulted in a much harder substrate of over 500 Hv which islikely to give rise to unacceptably low ductility for the core of anengineering component. The preferred composition is therefore in a rangearound the eutectic composition of Ti-10% Ni, typically 5 to 15% byweight nickel.

In a series of tests demonstrating the second aspect of the presentinvention, the lubricated sliding wear rates of five specimens werecompared using a modified Amsler wear testing machine. The flat surfaceto be tested was held stationary beneath the rotating outer rim of a 50mm diameter 8 mm wide disc of hardened steel rotating about a horizontalaxis. A contact load of 50 kgf was applied with a sliding speed of 0.52m/s and the wearing surfaces were lubricated by immersion in Tellus Oil37. The resulting rates of wear of the samples are shown in FIG. 5.

Sample No. 11 was untreated annealed Ti-6Al-4V and was observed to wearextremely rapidly. Sample No. 12 was Ti-8.5%Si in the as-cast state,without any surface re-melting, and also wore extremely rapidly. SampleNo. 13 was the same composition as Sample No. 12 but the surface hadbeen re-melted by electron beam using the same conditions as for SampleNo. 10, and the wear rate was reduced by a factor of more than ten.Sample No. 14 was again of the same composition, but the surface hadbeen treated by plasma nitriding in an atmosphere of 100% nitrogen on a40 kw plasma nitriding unit manufactured by Klockner Ionon GmbH for 12hours at 700° C., without any surface re-melting. The rate of wear wasimproved by a factor of over 100 compared with the untreated alloy(Sample No. 12). Sample No. 15 had been surface treated according to thesecond aspect of the present invention, namely by electron beam surfacere-melting without further alloying, followed by plasma nitriding in100% nitrogen for 12 hours at 700° C. in the same way as Sample No. 14.Sample No. 16 was again of the same composition as Samples 10 to 15 andhad again been surface treated according to the second aspect of thepresent invention, namely by electron beam surface re-melting withoutfurther alloying followed, in this instance by thermal oxidation in anair-circulation furnace for 10 hours at 650° C. It will be observed thatSample Nos. 15 and 16 were both treated in exactly the same way exceptthat, in step d) of the second aspect of the present invention, SampleNo. 15 was treated by plasma nitriding whereas Sample No. 16 was treatedby thermal oxidation. The wear rates of both Samples 15 and 16 werethereby reduced to a level less than that produced by either of the twocomponent processes on its own, and representing an improvement factorof several thousand compared with untreated material.

We claim:
 1. A method of forming a titanium alloy product having ahardened layer thereon, comprising the steps of:(1) forming anintermediate product from a titanium alloy consisting of (a) 8 to 11% byweight nickel, (b) 0 to 7% by weight of at least one strengtheningalloying element selected from the group consisting of aluminum, tin,zirconium, vanadium, chromium, iron, molybdenum and niobium, and (c) 0to 2% by weight of at least one alloying element which is asurface-property improver and which is selected from the groupconsisting of boron, carbon, nitrogen, oxygen and zirconium, the balanceapart from impurities being titanium, and (2) surface treating theintermediate product by a surface melting and rapid solidificationoperation so as to produce a titanium alloy product having a hardwear-resistant surface layer without substantially affecting the bulkproperties of the alloy.
 2. A titanium alloy product formed of atitanium alloy consisting of (a) 8 to 11% by weight nickel, (b) 0 to 7%by weight of at least one strengthening alloying element selected fromthe group consisting of aluminum, tin, zirconium, vanadium, chromium,manganese, iron, molybdenum and niobium, and (c) 0 to 2% by weight of atleast one surface property-improving element selected from the groupconsisting of boron, carbon, nitrogen, oxygen and zirconium, the balanceapart from impurities and incidental ingredients being titanium, thetitanium in the bulk of the product being present predominantly in the αphase, and said product having a layer thereon containing fine grainedTi-Ni eutectic.
 3. A method of forming a titanium alloy product which isresistant both to rolling contact fatigue and to scuffing, comprisingthe steps of:(a) forming a titanium alloy to the required product shape,(b) deep surface hardening the resultant shaped product to a depthgreater than 100 μm by a technique involving localized surfacere-melting without further alloying, so as to form a deep-hardenedlayer, (c) subsequent to step (b), optionally surface finishing to therequired final shape and/or surface finish, and (d) subsequently formingon said deep-hardened layer a surface film having a thickness which isnot greater than 100 μm and which is resistant to scuffing, said surfacefilm being selected from the group consisting of nitride, oxide, carbideand boride.
 4. A method as claimed in claim 3, wherein said titaniumalloy consists of (a) 2 to 15% by weight silicon or 5 to 15% by weightnickel, (b) 0 to 7% by weight of at least one strengthening alloyingelement selected from the group consisting of aluminum, tin, zirconium,vanadium, chromium, iron, molybdenum and niobium, and (c) 0 to 2% byweight of at least one alloying element which is a surface-propertyimprover and which is selected from the group consisting of boron,carbon, nitrogen, oxygen and zirconium, the balance apart fromimpurities and incidental ingredients being titanium, and wherein thedeep surface hardening step (b) is conducted by localized surfacere-melting.
 5. A method as claimed in claim 3, wherein the thickness ofsaid deep-hardened layer is 200 to 1000 μm.
 6. A method as claimed inclaim 3, wherein the thickness of said surface film is no more than 50μm.
 7. A method as claimed in claim 6, wherein the thickness of thesurface film is 1 to 20 μm.
 8. A method as claimed in claim 3, whereinthe forming step (d) comprises a plasma nitriding step in which thetitanium alloy is reacted with nitrogen in a low discharge plasma inorder to form layers of nitride and nitrogen-rich titanium on thesurface of the titanium alloy.
 9. A method as claimed in claim 3,wherein the forming step (d) comprises a thermal oxidation step in whichthe titanium alloy is heated in air at 600° to 850° C. to produce layersof oxide and oxygen-rich titanium on the surface of the titanium alloy.10. A method as claimed in claim 3, wherein said surface finishing step(c) is carried out.
 11. A method as claimed in claim 10, wherein thesurface finishing step (c) comprises machining or grinding to produce asmooth surface.
 12. A method as claimed in claim 3, further including(e) the step of performing a procedure after any of steps (b), (c) and(d) to modify the residual stresses in the material and/or its othermechanical properties.
 13. A method as claimed in claim 12, wherein step(e) is a shot peening or heat-treating step.
 14. A method as claimed inclaim 3, wherein the titanium alloy contains nickel in an amount of 8 to11% by weight.